1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a transmitter and a signal amplifier, and more particularly, to a transmitter and a signal amplifier using a Doherty structure without a 90° coupler.
2. Description of the Related Art
Complementary metal-oxide semiconductor (CMOS) technology contributes to reducing sizes of chips so as to be advantageous to integration and thus is most suitable for realizing system on a chip (SoC). A radio frequency (RF) performance becomes higher with the reduction in the sizes of the chips. However, a 90° coupler inappropriate for integrated circuit (IC) integration is required to be used in a transmitter using a Doherty structure. Thus, it is difficult to manufacture monotype chips.
FIG. 1 is a schematic block diagram of a related art transmitter using a Doherty structure. Referring to FIG. 1, the related art transmitter includes a digital-to-analog converter (DAC) 10, a base band analog filter 20, a mixer 30, a 90° coupler 40, a main amplifier 50, an auxiliary amplifier 60, and a 90° offset line 70.
The DAC 10 converts a predetermined digital signal into an analog signal.
The base band analog filter 20 filters the connected analog signal by the DAC 110 to remove a high frequency noise component so as to pass only a base band signal and amplifies the base band signal with a gain to provide the amplified signal to the mixer 30.
The mixer 30 mixes the signal transmitted from the base band analog filter 20 with a high frequency local resonator signal generated by a local resonator signal generator (not shown) to output a high frequency signal.
The 90° coupler 40 divides the high frequency signal into a first high frequency signal and a second high frequency signal which is 90° out of phase with the first high frequency signal and outputs the first and second high frequency signals to the main amplifier 50 and the auxiliary amplifier 60, respectively.
The main and auxiliary amplifiers 50 and 60 respectively amplify the first and second high frequency signals with a predetermined gain and output the amplified first and second high frequency signals.
The 90° offset line 70 delays by 90° a phase of the first high frequency signal amplified by the main amplifier 50 so that the first high frequency signal is in phase with the second high frequency signal.
An output signal obtained by coupling the first and second high frequency signals respectively output from the main and auxiliary amplifiers 50 and 60 through the above-described process additionally undergoes impedance matching and noise removing processes and then is transmitted through an antenna (not shown) to a receiver (not shown).
As described above, the Doherty structure divides the high frequency signal output from the mixer 30 into the first and second high frequency signals 90° out of phase with each other and provides the first and second high frequency signals the main and auxiliary amplifiers 50 and 60 in parallel to amplify the first and second high frequency signals.
FIG. 2 is a graph illustrating an output power with respect to an input power in a Doherty structure. Referring to FIG. 2, the Doherty structure generally uses an amplifier amplifying an input power in a full band or saturating an output power if the input power is equal to or more than a predetermined threshold value, for example, an A class or AB class amplifier as a main amplifier and a C class amplifier turned off at a low input voltage and then turned on at a voltage equal to or greater than a predetermined threshold value to amplify an input voltage as an auxiliary amplifier. Thus, the Doherty structure couples an output A1 of the main amplifier to an output A2 of the auxiliary amplifier to produce an output A1+A2 so as to have a highly linear and efficient operation characteristic as shown in FIG. 2.
However, the Doherty structure requires the 90° coupler 40 dividing the high frequency signal output from the mixer 30 into the first and second high frequency signals which are 90° out of phase with each other and providing the first and second high frequency signals to the main and auxiliary amplifier 50 and 60. Further, in the Doherty structure, the 90° coupler 40 is formed on a printed circuit board (PCB) using a transmission line and thus cannot be realized as a an IC or occupies a large area. For example, in a band of 900 MHz, a length of a wavelength is 18.7 cm and. Thus, the 90° coupler 40 cannot be realized as an IC.
The DAC 10, the base band analog filter 20, the mixer 30, and the main and auxiliary amplifiers 50 and 60 are manufactured through a CMOS process. However, the 90° coupler 40 cannot be manufactured through the CMOS process due to its size. Thus, the Doherty structure cannot be realized as a single chip RFIC.
In addition, the 90° coupler 40 formed on the PCB using the transmission line divides a signal with a narrow frequency band into first and second high frequency signals which are 90° out of phase with each other.